Compositions of particulate magnetic oxides with a defect spinel structure, preparation thereof and application thereof

ABSTRACT

Particulate magnetic oxide compositions based on iron oxide (III) and oxide of at least one bivalent metal selected among cobalt, iron, copper, zinc, magnesium, nickel, manganese and cadmium, characterized in that said compositions contain additionally in the form of an oxide from 0.2 to 5% by weight calculated on the total weight at least one additive or substituent selected among alkali metals, alkaline-earth metals, boron, the elements of columns 3, 4 and 5 of the periodical classification of elements having a molecular mass higher than 26, the transition metals 3d and 4d other than those already mentioned, and the rare earths, with the proviso that when an alkali metal or tin is present, at least another additive or substituent is also present, and in that said compositions have a structure of the lanucar spinel type; the preparation and application thereof are also disclosed.

The object of this invention is new compositions of particulate magneticoxides with a defect spinel structure, preparation thereof andapplication thereof.

It is known that in the industry relating to materials for high-densitymagnetic recording, powdered oxides of ferromagnetic materials are usedin the form of submicron acicular particles with a high coercive fieldand high saturation and remanent magnetization values.

These magnetic powders can be prepared starting with organic salts, inparticular, starting with mixed oxalates.

In French Patent Application No. 72.14215 (Publication No. 2,180,575), aprocess is proposed for preparing magnetic oxides of the ferrite type,i.e., compounds based on iron sesquioxide in which some of the ferricions and some of the defects are replaced by one or more bivalentcations.

The bivalent cations which are compatible with a cubic spinel structureare those of cobalt, iron, zinc, copper, magnesium, nickel, manganeseand cadmium.

The process according to the above-mentioned French patent consists,starting from a mixed oxalate, of decomposing this oxalate by heating itin air to a moderate temperature to produce a superparamagnetic oxide(because of the very small dimensions of the crystals, on the order of afew tens of Angstroms), reducing this oxide with hydrogen in thepresence of water vapor to steer the crystallization of the oxide, whichis still poorly organized, towards a cubic spinel lattice and produce amagnetite substituted with one or more bivalent metals, and finallyoxidizing the magnetite thus obtained to convert all of the iron to thetrivalent state. This results in solid solutions of gamma-Fe₂ O₃ andferrites of the type MOFe₂ O₃, where M represents one or more bivalentmetals selected from those previously mentioned.

The process for converting oxalates into oxides which has just beendescribed makes it possible to produce particles of magnetic oxides mostof which have retained the shape and dimensions of the initial oxalateparticles. However, this process is not entirely satisfactory, forreasons which will be indicated.

It is worth mentioning here that when oxide particles are prepared,well-formed domains of matter (crystallites) form within the particles,separated by transition zones (crystallite boundaries) and micropores.

In this application, the term texture will be used to designate thesearrangements of matter within a particle, the term internal sinteringwill be used to indicate improvement in this texture under the influenceof heat treatments, and external sintering will be understood in itsusual sense of leading to welding among particles with the appearance ofdendrites. Internal sintering makes it possible to improve magneticperformance, while external sintering, in contrast, leads to asignificant degradation therein.

Thus, the process which has just been described is not satisfactory interms of the texture of the resulting oxide particles. The particleshave a granular structure and exhibit crystallite boundaries with thepresence of dendrites.

The presence of crystallite boundaries has an unfavorable influence notonly on the magnetic properties but also on the mechanical strengthcharacteristics of the particles. Specifically, crystallite boundariesconstitute breakage point nuclei for the grains which, during subsequenthandling of the powders, have a tendency to shatter into granules whichare often essentially spherical in shape and are not compatible withgood magnetic properties.

The powders obtained after such handling are therefore relativelyheterogeneous.

In summary, these texture defects do not make it possible to obtainoxide powders meeting current needs in the area of magnetic recording.

The object of this invention is compositions of particulate magneticoxides containing new adjuvant or substituent elements.

The adjuvants promote internal sintering and make it possible tosignificantly improve the texture of the particles and prevent externalsintering. The particles produced have a homogeneous and nongranulartexture with an absence of dendrites. This homogeneity of textureresults in a considerable decrease in background noise.

The introduction of new substituent elements, making it possible tomodify the magnetic properties and improve chemical stability, provide agreat deal of latitude for adapting and modulating the properties ofmaterials depending on requirements.

In addition, thanks to improvements made to the process for preparingthe starting oxalates (improvements which do not form part of theinvention claimed here) and thanks also to improvements made to theprocess for converting the oxalates into oxides, it has been possible toproduce, in a controlled and reproducible manner, new compositions ofoxides having suitable particle dimensions and satisfactory thermal andchemical stability.

It is worth noting that the various improvements forming the object ofthis invention have made it possible to introduce, into the crystallattice of the oxides, ions such as ions of alkaline earth metals, rareearths or potassium, while it is generally considered that ions whoseradius is equal to or greater than one Angstrom cannot enter into acrystal lattice of the spinel type such as that of gamma-Fe₂ O₃ ; see,for example, Pascal: Traite de chimie minerale volume XVII, pages629-630.

Integration of these ions into the crystal lattice can be demonstratedby the increase in the transformation temperature of the crystal systemdetected by differential thermal analysis. Specifically, it is knownthat the transformation temperature of gamma-Fe₂ O₃ is approximately460° C. With the substitution of cobalt and zinc in a total amount of 5%by weight, this transformation temperature rises to approximately 560°C. As the experimental section below shows, the addition of barium, forexample, makes possible a very sharp increase in the transformationtemperature, and this result in fact shows that the barium ion becomesintegrated into the crystal lattice, whose stability it increases.

Furthermore, it must be noted that it is also not necessary for theadded element to be in the crystal lattice of the iron-based oxide,since the specific surface area and the texture can be modified by anelement which is located outside the crystal lattice. This isparticularly the case with potassium, which, above 2% by weight, nolonger enters into the crystal lattice but does significantly influencethe crystal size and the specific surface area.

The object of this invention is therefore new compositions ofparticulate magnetic oxides containing iron(III) oxide and the oxide ofat least one bivalent metal selected from cobalt, iron, copper, zinc,magnesium, nickel, manganese and cadmium, characterized by the fact thatthe said compositions additionally contain, in the form of oxide, at aproportion of between 0.2 and 5% by weight referred to the total weight,at least one adjuvant or substituent selected from among the alkalimetals (especially lithium, sodium, potassium), the alkaline earthmetals (calcium, barium, strontium), boron, the elements in columns 3, 4and 5 of the periodic table of the elements having a molecular weightgreater than 26, the 3d and 4d transition metals other than thosealready mentioned and the rare earths, with the understanding that whenan alkali metal or tin is present, at least one other adjuvant orsubstituent is also present, and by the fact that the said compositionshave a defect spinel structure.

The percentage by weight indicated concerns the adjuvant or substituentelement and not the corresponding oxide.

The elements in columns 3 through 5 of the periodic table with amolecular weight greater than 26 are, in particular: aluminum, silicon,phosphorus, gallium, germanium, arsenic, indium, antimony, bismuth andlead.

The 3d transition metals whose presence can constitute a characteristicof compositions according to the invention are: scandium, titanium,vanadium and chromium.

The 4d transition metals whose presence can constitute a characteristicof compositions according to the invention are: yttrium, zirconium,niobium and molybdenum.

Among the rare earths which can be present in compositions according tothe invention, those mentioned in particular are neodymium,praseodymium, samarium, europium, gadolinium, terbium, dysprosium,holmium, erbium, ytterbium, lutetium, cerium, thulium, etc.

Generally, the bivalent metal which is present in the compositionaccording to the invention represents between 1 and 10%, preferablybetween 1 and 5% by weight of the said composition.

The low levels of substituents or adjuvants present in the compositionsaccording to the invention make it difficult to characterize thesecompositions by studing changes in the parameters of the crystal latticeby X-ray diffraction. The best criterion for characterizing theintroduction of substituents into the crystal lattice is the change inthe gamma→alpha transformation temperature, which varies greatly withthe nature and concentration of the substituent.

The particulate compositions according to the invention are, especially,compositions formed of acicular particles with dimensions of severalmicrons or more, and in particular compositions in which the particleshave lengths between 0.05 and 0.5 μm, with an acicular ratio between 2and 5, use of which is recommended for magnetic recording.

Among the particulate compositions according to the invention,particular mention is made of those in which at least 80% of theparticles have a length equal to the average length ±0.1 μm, and anacicular ratio between ca. 2 and 5, with the average length beingbetween 0.15 and 0.35 μm.

Such compositions can be obtained by selecting the starting compositionsof oxalates in which the particles have suitable size and shapecharacteristics, taking into account the fact that when the oxalates areconverted into oxides with the process according to the invention, thelength of the particles is divided by a number which can vary betweenapproximately 1.5 and 2, and the acicular ratio is divided by a numberwhich can vary between approximately 1.5 and 2.5.

The process for preparing the starting oxalates and the finalparticulate magnetic oxides even makes it possible to preparecompositions in which at least 85% of the particles, and even in certaincases more than 90% of the particles, have these dimensionalcharacteristics.

One particular object of the invention is particulate compositions aspreviously defined, whose chemical compositions corresponds to theformula:

    (1-z)Fe.sub.2 O.sub.3,zMFe.sub.2 O.sub.4,yM'.sup.(n) O.sub.n/2

in which:

M represents at least one bivalent metal selected from cobalt, iron,zinc, copper, magnesium, nickel, manganese and cadmium,

M' represents a substituent or adjuvant selected from those mentionedabove, with the understanding that when an alkali metal is present, atleast one other adjuvant or substituent is also present,

n is the valence of M',

z represents the number of moles of bivalent metal M, z being such thatthe bivalent metal represents between 1 and 10%, preferably between 1and 5% of the total weight of the composition, and

y represents the number of moles of substituent and/or adjuvant M', ybeing such that M' represents between 0.2 and 5% by weight of thecomposition.

Of course, when several substituents or dopants M' are present, therespective valences and proportions of the elements M' must be takeninto account to calculate the quantity of oxygen represented by yn/2.

For example, if yM'.sup.(n) O_(n/2) represents two oxides of theelements M'.sub.(1) and M'.sub.(2), with valences n₁ and n₂,respectively, present in the respective molar proportions y₁ and y₂(where y₁ +y₂ =y), then yn/2 is expressed by: ##EQU1##

Another object of the invention is a process for preparing theparticulate compositions as defined above.

This process, comprising steps consisting of decomposing the startingmixed oxalate by heating it in air, reducing the resulting product in ahydrogen atmosphere and then oxidizing the reduced product by heating itin air, is characterized by the fact that before the reduction step, theoxides obtained after the decomposition step are subjected to a heattreatment in an oxidizing atmosphere at a temperature of 550°-700° C.for a period which can vary between approximately ten minutes and fivehours.

This heat treatment is performed, for example, in air.

The temperature rise for this heat treatment must be relatively rapid;it is preferably 150°-300° C. per hour.

The purpose of this heat treatment step is internal sintering of theelementary grains, with densification of the particles, and making itpossible to prevent the formation of dendrites or particles with agranular texture.

The optimum temperature and treatment time are determined, in each case,by observing samples of the treated composition with an electronmicroscope. The conditions to be used are of course those which lead tothe production of particles in which the crystals have the largestdimensions, that is, in practice, particles in which crystalliteboundaries are seen to disappear or substantially decrease in number.The temperature and the treatment time must nonetheless be sufficientlylow to prevent the appearance of external sintering.

At the end of the heat treatment, a quenching step is performed, forexample, quenching in air.

In the starting mixed oxalate, the elements other than carbon and oxygenare generally present in the proportions desired in the final oxide.

However, it is also possible to mix the adjuvants and even thesubstituent elements into the starting oxalate as additives. Forexample, the oxalate can be impregnated with a solution of the adjuvantand/or substituent elements in an appropriate solvent. In this case, thesubstituent or even adjuvant elements are primarily present only in thesurface layers of the final oxide particles.

In particular embodiments, the process according to the invention canalso present the following characteristics, taken in isolation or incombination.

The oxalic precursors and any additives are decomposed in an oxidizingatmosphere at a temperature between 180° and 300° C. with a slowtemperature rise, for example, less than 20° C. per hour when working inair; this decomposition step is preferably performed on the startingmaterial present as a thin layer, so that good temperature homogeneityis obtained with air scavenging; the more oxidizing the atmosphere, theslower the temperature rise must be.

The reduction step is performed in an atmosphere consisting of a mixtureof hydrogen and an inert gas containing between 8 and 30% by volume ofhydrogen and containing no water vapor, or containing less than 3% byvolume thereof; this is because it has been found that the presence ofwater in excessive proportions caused excessively rapid growth of thecrystals within the needles and therefore caused them to burst and breakinto spherical particles which often had excessively small dimensionsand did not have the required geometrical anisotropy; the inert gas is,for example, nitrogen.

Reduction occurs at a temperature greater than 300° C., preferably at atemperature of 320°-500° C., until the alpha-Fe₂ O₃ phase disappears;the heating time should be as short as possible (generally one to twohours) to prevent external sintering; the reduction step is preferablyfollowed by quenching, for example, in air. Disappearance of thealpha-Fe₂ O₃ phase can be observed by X-ray diffraction.

The final oxidation step is performed by heating in air at a temperatureof 100°-500° C. with a relatively rapid temperature rise, for example,150° C. per hour; it is preferable to proceed at the minimum temperaturesufficient to allow the desired oxidation of the composition within anacceptable period, for example, on the order of approximately two hours;rapid cooling then follows, for example, by means of air quenching.

The process according to the invention can comprise, if desired, asupplementary step in cases where one wishes to increase the size of thecrystals. To do so, at the end of the reduction stage, instead ofperforming quenching, the composition is subjected to a heat treatmentin an inert atmosphere (i.e., one which is neither oxidizing norreducing, for example, under nitrogen or dry argon) at a temperaturebetween approximately 450° and 700° C. The temperature rise is, forexample, on the order of 150° to 250° C./hour. This sort of heattreatment promotes crystal growth and the reduction of porosity. In eachcase, the optimum treatment temperature and time are determined bymonitoring the resulting change in crystal size by observing the widthof lines on X-ray diffraction spectra. At the end of this heattreatment, quenching is performed, for example, in air.

It is easy to introduce Fe(II) as a substituent in the last step of theprocess, during which the magnetites are oxidized. To prevent completeoxidation of Fe(II) to Fe(III), either oxidation can be performed at alow temperature (for example, at 100°-250° C.), or oxygen consumptioncan be limited by controlling the composition of the atmosphere in theenclosure. In other cases, the oxidation step is performed at a highertemperature (200°-500° C.).

Introduction of Fe(II) into the crystal lattice is of interest since itis the only inexpensive substituent with a positive magnetostrictioncoefficient. It can especially be combined with cobalt, whosemagnetostriction coefficient is negative.

The oxalate compositions used as starting products in the processaccording to the invention can be prepared according to known processes.They can also be prepared by a process characterized by the fact:

that, on the one hand, a solution is prepared containing 0.1 to 1 moleper liter of oxalic acid in a first organic solvent having a dielectricconstant less than 30, with the said solution containing a maximum of15% water by volume;

that, on the other hand, a concentrated solution of at least one mineralsalt of the metal or metals which one wishes to produce in the form ofan oxalate is prepared in a liquid medium containing at least 25% byvolume of a second organic solvent chosen from methanol, ethanol,tetrahydrofuran and the polyols which are liquid at ambient temperatureor mixtures of these solvents, with any remainder of the said liquidmedium consisting of water, with the total concentration of the saltsbeing greater than one mole/liter, with the said solution beingacidified to promote solubilization of the salts and furthermorepossibly containing dopants;

then, that the said salt solution is progressively poured into the saidoxalic acid solution while agitating the latter to produce a particulateoxalate precipitate;

with the understanding that at least one of the said organic solvents isan alcohol.

This latter process, which does not form part of the invention claimedhere, is especially characterized by the fact that minimal quantities ofwater are used in the starting solutions of salts and oxalic acid. Inaddition, this process makes it possible to control the dimensions ofthe particles by selecting the solvents, with the use of solvents with alow dielectric constant and the absence of water (or the presence ofsmall quantities of water) promoting the production of particles ofreduced length.

The oxide particles obtained by means of the process according to theinvention are dimensionally quite homogeneous and have interestingmagnetic properties enabling them to be used in the production ofmaterials for magnetic recording, for example, for high-density magneticrecording. This use also forms part of the invention.

The following examples illustrate the invention, although withoutlimiting it. These examples more specifically illustrate the preparationof magnetic powders suitable for high-density recording.

EXAMPLE 1

The starting product used is a composition of acicular mixed oxalateparticles with the formula:

    (Fe.sub.0,9391 Co.sub.0,0420 Zn.sub.0,0124 Ba.sub.0,0064).sup.2+ C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

Ten grams of this oxalate are decomposed in a tubular furnace which isheated to a temperature of 300° C. with a temperature rise of 20° C. perhour.

A sintering operation is then performed by bringing the resulting oxideat a rate of 300° C. per hour to a temperature of 600° C., at which itis kept for 15 minutes. Air quenching is performed to retain theacicular shape of the particles and to produce a nongranular internaltexture with no dendrites.

The resulting oxide is then reduced by heating it in an N₂ /H₂atmosphere (90/10) at a rate of 150° C. per hour to a temperature of350° C., which is maintained for two hours. After air cooling, theresulting product is oxidized by heating in air with a temperature riseof 150° C. per hour to a temperature of 430° C. which is maintained fortwo hours. Cooling is performed by quenching in air.

The resulting product is in the form of acicular particles with anaverage length of 0.37±0.006 μm and an average diameter of 0.045±0.09μm.

The structure of this oxide, studied by X-ray diffraction, is of thedefect spinel type deriving from gamma-Fe₂ O₃.

The particles obtained have the following composition (by weight):

Co=3.08%

Zn=1.01%

Ba=1.10%

Values for magnetic properties are as follows:

H_(c) =780 Oe

σR=39.9 u.e.m./g

Differential thermal analysis: transformation temperature=650° C.

The starting mixed oxalate, preparation of which does not form part ofthe invention, was prepared as follows.

A solution A of 0.5 molar oxalic acid is prepared by dissolving 13.75 gof oxalic acid in 95% by volume of ethyl alcohol at ambient temperature.

A twofold molar solution B is prepared by dissolving, in a mixturecontaining 60% water and 40% ethylene glycol, the following salts:

FeCl₂,4H₂ O: 18.7 g

CoCl₂,6H₂ O: 1 g

ZnCl₂ : 0.5 g

BaCl₂ : 0.8 g

0.5 cm³ of a 12N aqueous solution of hydrochloric acid is added to themixture.

Solution B is added to solution A with vigorous agitation, by atomizingsolution B at a rate of 7 liters/hour over solution A, using an atomizerproducing droplets with an average size of 0.8 mm.

The precipitated oxalate particles are washed and dried.

EXAMPLE 2

The procedure is analogous to that described in Example 1, starting witha mixed oxalate with the formula:

    (Fe.sub.0,9393 Co.sub.0,0430 Zn.sub.0,0147 Ba.sub.0,0030).sup.2+ C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

The final oxide possesses morphological and structural propertiesidentical to those of the oxide obtained in Example 1.

The particles obtained have the following composition (by weight):

Co=3.16%

Zn=1.2%

Ba=0.51%

Values for magnetic properties are as follows:

H_(c) =740 Oe

σR=43.9 u.e.m./g

The starting oxalate is prepared in a manner similar to the preparationof the starting oxalate in Example 1, using suitable proportions offerrous chloride, cobalt chloride, zinc chloride and barium chloride.

EXAMPLE 3

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,9335 Co.sub.0,0433 Zn.sub.0,0108 Ba.sub.0,0124).sup.2+ C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

The starting oxalate particles had the following characteristics:

average length: 0.29 μm

average diameter: 0.042 μm

average acicular ratio: 6.9.

The oxide particles obtained have the following characteristics:

average length: 0.21 μm

average diameter: 0.045 μm

average acicular ratio: 4.7.

The particles obtained have the following composition (by weight):

Co=3.16%

Zn=0.87%

Ba=2.1%

Values for magnetic properties are as follows:

H_(c) =751 Oe

σR=36 u.e.m./g

Differential thermal analysis: transformation temperature=740° C.

The starting oxalate is prepared in a manner analogous to that describedin Example 1.

EXAMPLE 4

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,9450 Co.sub.0,0437 Ba.sub.0,0112).sup.2+ C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

The starting oxalate particles had the following characteristics:

average length: 0.30 μm

average diameter: 0.045 μm

average acicular ratio: 6.7.

The oxide particles obtained have the following characteristics:

average length: 0.19 μm

average diameter: 0.047 μm

average acicular ratio: 4.

The particles obtained have the following composition (by weight):

Co=3.19%

Ba=1.91%

Values for magnetic properties are as follows:

H_(c) =800 Oe

σR=40.5 u.e.m./g

Differential thermal analysis: transformation temperature=730° C.

The starting oxalate is prepared in a manner analogous to that describedin Example 1.

EXAMPLE 5

The procedure is analogous to that described in Example 1, except thestarting product is a mixed oxalate doped with boron (in the form ofboric acid) with the following formula:

    (Fe.sub.0.9342 Co.sub.0.0435 Zn.sub.0.0112 Ba.sub.0.0111).sup.2+ C.sub.2 O.sub.4.sup.2- 2·H.sub.2 O→0.0249 mole of H.sub.3 BO.sub.3

The sintering treatment in air is performed at 630° C. for 30 minutes.

The reduction treatment is performed at 430° C. for four hours 30minutes.

The oxide obtained is in the form of acicular particles with an averagelength of 0.33±0.06 μm and an average diameter of 0.040±0.009 μm.

The structure is of the defect spinel type, indentical to that ofgamma-Fe₂ O₃.

The composition of the oxide is as follows:

Co=3.14%

Zn=0.90%

Ba=1.86%

B=0.33%

The magnetic properties are as follows

H_(c) =670 Oe

σR=33.7 u.e.m./g

Differential thermal analysis: transformation temperature=780° C.

The starting oxalate is prepared in a manner analogous to that describedin Example 1. This oxalate is then brought into contact with an aqueoussolution of 3% boric acid by weight with 500 cm³ of solution per 15grams of oxalate.

EXAMPLE 6

The procedure is as in Example 5, but after the reduction step, thehydrogen supply is cut off and the composition is heated under nitrogento 610° C. for three hours with a temperature rise on the order of 200°C. per hour. Air cooling is performed and then the oxidation step isperformed as in Example 5.

This results in a composition analogous to that in Example 5, but thanksto the supplementary heat treatment, the average dimensions of thecrystals are 550 Angstroms, rather than 300 Angstroms for thecomposition in Example 5.

EXAMPLE 7

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,9300 Co.sub.0,0437 Zn.sub.0,0263).sup.2+.sub.0,9929 Dy.sup.3+.sub.0,0047 C.sub.2 O.sub.4.sup.2-,2H.sub.2 O,

The final oxide has morphological and structural characteristicsidentical to those of the oxide obtained in Example 1.

The particles obtained have the following composition (by weight):

Co=3.19%

Zn=2.13%

Dy=0.96%

Values for magnetic properties are as follows:

H_(c) =670 Oe

σR=36 u.e.m./g

Differential thermal analysis: transformation temperature=590° C.

The starting oxalate is prepared in a manner analogous to that describedin Example 1. Dysprosium is introduced in the form of the chlorideDyCl₃.

EXAMPLE 8

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,9368 Co.sub.0,0432 Zn.sub.0,0135 Ba.sub.0,0065).sup.2+.sub.0,9907 Dy.sup.3+.sub.0,0062 C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

The final oxide has morphological and structural characteristicsidentical to those of the oxide obtained in Example 1.

The particles obtained have the following composition (by weight):

Co=3.12%

Zn=1.08%

Dy=1.25%

Ba=1.22%

Values for magnetic properties are as follows:

H_(c) =710 Oe

σR=38 u.e.m./g

Differential thermal analysis: transformation temperature=720° C.

The starting oxalate is prepared in a manner analogous to that describedin Example 7.

EXAMPLE 9

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,9253 Co.sub.0,0442 Zn.sub.0,0264 Yb.sub.0,0041).sup.2+ C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

The final oxide has morphological and structural characteristicsidentical to those of the oxide obtained in Example 1.

The particles obtained have the following composition (by weight):

Co=3.24%

Zn=2.15%

Yb=0.88%

Values for magnetic properties are as follows:

H_(c) =735 Oe

σR=38 u.e.m./g

Differential thermal analysis: transformation temperature=570° C.

The starting oxalate is prepared in a manner analogous to that describedin Example 1. Ytterbium is introduced in the form of the chloride YbCl₂.

EXAMPLE 10

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,9296 Co.sub.0,0438 Zn.sub.0,0266).sup.2+.sub.0,9927 Gd.sup.3+.sub.0,0049 C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

The final oxide has morphological and structural characteristicsidentical to those of the oxide obtained in Example 1.

The particles obtained have the following composition (by weight):

Co=3.20%

Zn=2.11%

Gd=0.96%

Values for magnetic properties are as follows:

H_(c) =673 Oe

σR=35 u.e.m./g

Differential thermal analysis: transformation temperature=610° C.

The starting oxalate is prepared in a manner analogous to that describedin Example 1. Gadolinium is introduced in the form of the chlorideGdCl₃,6H₂ O.

EXAMPLE 11

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,9147 Co.sub.0,0482 Zn.sub.0,0334 Eu.sub.0,00375).sup.2+ C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

The final oxide has morphological and structural characteristicsidentical to those of the oxide obtained in Example 1.

The particles obtained have the following composition (by weight):

Co=3.54%

Zn=2.72%

Eu=0.71%

Values for magnetic properties are as follows:

H_(c) =758 Oe

σR=38.5 u.e.m./g

The starting oxalate is prepared in a manner analogous to that describedin Example 1. Europium is introduced in the form of the nitrateEu(NO₃)₂.

EXAMPLE 12

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0.9237 Co.sub.0.0438 Zn.sub.0.0325).sup.2+.sub.0.9972 Sb.sub.0.0018.sup.3+ C.sub.2 O.sub.4.sup.2-.2H.sub.2 O

The final oxide has morphological and structural characteristicsidentical to those of the oxide obtained in Example 1.

The particles obtained have the following composition (by weight):

Co=3.32%

Zn=2.65%

Sb=0.28%

Values for magnetic properties are as follows:

H_(c) =720 Oe

σR=27 u.e.m./g

The starting oxalate is prepared in a manner analogous to that describedin Example 1. Antimony is introduced in the form of the chloride SbCl₃.

EXAMPLE 13

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,9336 Co.sub.0,0390 Zn.sub.0,0274).sup.2+.sub.0,9688 V.sup.3+.sub.0,0131 Ti.sup.4+.sub.0,0058 C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

The final oxide has morphological and structural characteristicsidentical to those of the oxide obtained in Example 1.

The particles obtained have the following composition (by weight):

Co=2.83%

Zn=2.20%

V=0.85%

Ti=0.35%

Values for magnetic properties are as follows:

H_(c) =550 Oe

σR=33.6 u.e.m./g

The starting oxalate is prepared in a manner analogous to that describedin Example 1. Titanium is introduced in the form of TiCl₃ and vanadiumin the form of VCl₃.

EXAMPLE 14

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,9324 Co.sub.0,0456 Zn.sub.0,0221).sup.2+.sub.0,9561 Ga.sup.3+.sub.0,0216 Ti.sup.4+.sub.0,0058 C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

The final oxide has morphological and structural characteristicsidentical to those of the oxide obtained in Example 1.

The particles obtained have the following composition (by weight):

Co=3.26%

Zn=1.75%

Ga=1.91%

Ti=0.16%

Values for magnetic properties are as follows:

H_(c) =660 Oe

σR=36.7 u.e.m./g

The starting oxalate is prepared in a manner analogous to that describedin Example 1. Gallium is introduced in the form of the sulfate Ga₂(SO₄)₃, and titanium in the form of TiCl₃.

EXAMPLE 15

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,9438 Co.sub.0,0412 Zn.sub.0,0150).sup.2+.sub.0,9500 Al.sup.3+.sub.0,0334 C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

The final oxide has morphological and structural characteristicsidentical to those of the oxide obtained in Example 1.

The particles obtained have the following composition (by weight):

Co=2.97%

Zn=1.20%

Al=1.16%

Values for magnetic properties are as follows:

H_(c) =635 Oe

σR=35.4 u.e.m./g

The starting oxalate is prepared in a manner analogous to that describedin Example 1. Aluminum is introduced in the form of the chloride AlCl₃.

EXAMPLE 16

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,9304 Co.sub.0,0433 Zn.sub.0,0263).sup.2+ C.sub.2 O.sub.4.sup.2-,2H.sub.2 O

The final oxide has morphological and structural characteristicsidentical to those of the oxide obtained in Example 1.

The particles obtained have the following composition (by weight):

Co=3.16%

Zn=2.20%

Values for magnetic properties are as follows:

H_(c) =665 Oe

σR=43.3 u.e.m./g

Differential thermal analysis: transformation temperature=590° C.

The starting oxalate is prepared in a manner analogous to that describedin Example 1.

EXAMPLE 17

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,912 Co.sub.0,043 Zn.sub.0,012 Ba.sub.0,006 Mn.sub.0,027).sup.2+ C.sub.2 O.sub.4.sup.2-,2H.sub.2 O.

The oxide particles obtained have the following composition (by weight):

Co=3.16%

Zn=0.94%

Ba=1.10%

Mn=1.84%

Magnetic properties:

H_(c) =691 Oe

σR=43.9 u.e.m./g.

EXAMPLE 18

The procedure is analogous to that described in Example 5, starting witha mixed oxalate of the formula:

    (Fe.sub.0,953 Co.sub.0,038 Ba.sub.0,009).sup.2+ C.sub.2 O.sub.4,.sup.2-,2H.sub.2 O+0,043 mole de H.sub.3 BO.sub.3.

However, the final oxidation treatment is performed at a temperature of220° C. (rather than 430° C.) for two hours, which makes it possible tokeep some of the iron in the form of iron(II).

The starting oxalate particles had the following characteristics:

average length: 0.33 μm

average diameter: 0.042 μm

average acicular ratio: 7.9.

The oxide particles obtained have the following characteristics:

average length: 0.22 μm

average diameter: 0.044 μm

average acicular ratio: 5.

The oxide particles obtained have the following composition (by weight):

Co=2.81%

Ba=1.49%

B=0.59%

Fe²⁺ =30% of total iron

Magnetic properties:

H_(c) =800 Oe

σR=36.2 u.e.m./g

Transformation temperature=740° C.

EXAMPLE 19

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,922 Co.sub.0,035 Ca.sub.0,043).sup.2+ C.sub.2 O.sub.4.sup.2-,2H.sub.2 O

The oxide particles obtained have the following composition (by weight):

Co=2.63%

Ca=2.18%

Magnetic properties:

H_(c) =542 Oe

σR=44.5 u.e.m./g

Transformation temperature: 720° C.

The starting oxalate particles had the following characteristics:

average length: 0.28 μm

average diameter: 0.04 μm

average acicular ratio: 7.

The oxide particles obtained have the following characteristics:

average length: 0.19 μm

average diameter: 0.043 μm

average acicular ratio: 4.4.

EXAMPLE 20

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,951 Co.sub.0,036 Sr.sub.0,013).sup.2+ C.sub.2 O.sub.4.sup.2-,2H.sub.2 O

The particles obtained have the following composition (by weight):

Co=2.68%

Sr=1.42%

Magnetic properties:

H_(c) =642 Oe

σR=44.7 u.e.m./g

Transformation temperature: 780° C.

EXAMPLE 21

The procedure is analogous to that described in Example 1, starting witha mixed oxalate of the formula:

    (Fe.sub.0,941 Co.sub.0,037 Cd.sub.0,023).sup.2+ C.sub.2 O.sub.4.sup.2-,2H.sub.2 O

The particles obtained have the following composition (by weight):

Co=2.56%

Cd=0.023%

Magnetic properties:

H_(c) =683 Oe

σR=40.2 u.e.m./g

Transformation temperature: 625° C.

We claim:
 1. A particulate magnetic oxide composition made of particleshaving a defect spinel structure, consisting essentially of (i) iron(III) oxide, (ii) an oxide of at least one bivalent metal selected fromthe group consisting of cobalt, iron, copper, zinc, magnesium, nickel,manganese and cadmium, and (iii), in the form of an oxide thereof, atleast one adjuvant or substituent selected from the group consisting ofan alkali metal, an alkaline earth metal, boron, aluminum, gallium,germanium, tin, arsenic, indium, antimony, bismuth, lead, the 3d and 4dtransition metals other than those already mentioned and a rare earthelements, wherein the weight of said at least one bivalent metalrepresents between 1 and 10 percent of the weight of said composition,and wherein the weight of said at least one adjuvant or substituentrepresents between 0.2 and 5 percent of the weight of said composition,with the proviso that when an alkali metal or tin is present, at leastone other adjuvant or substituent is also present.
 2. The composition ofclaim 1 wherein the weight of said at least one bivalent metalrepresents between 1 and 5 percent of the total weight.
 3. Thecomposition of claim 1 wherein said 3d transition metal is selected fromthe group consisting of scandium, titanium, vanadium and chromium. 4.The composition of claim 1 wherein said 4d transition metal is selectedfrom the group consisting of yttrium, zirconium, niobium and molybdenum.5. The composition of claim 1 wherein said rare earth is selected fromthe group consisting of neodymium, praseodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, ytterbium, lutetium,thulium and cerium.
 6. The composition of claim 1 having a chemicalcomposition corresponding to the formula:

    (1-z)Fe.sub.2 O.sub.3,zMFe.sub.2 O.sub.4,yM'.sup.(n) O.sub.n/2

wherein M represents at least one bivalent metal selected from the groupconsisting of cobalt, iron, zinc, copper, magnesium, nickel, manganeseand cadmium; M' represents a substituent or adjuvant as defined in claim1, with the proviso that when an alkali metal or tin is present at leastone other adjuvant or substituent is also present; n is the valence ofM'; z represents the number of moles of said bivalent metal M, z beingsuch that said bivalent metal represents between 1 and 10 percent of thetotal weight of said composition; and Y represents the number of molesof M', y being such that M' represents between 0.2 and 5 percent byweight of said composition.
 7. The composition of claim 1 wherein saidparticles have lengths between 0.05 and 0.5 μm with an acicular ratiobetween 2 and
 5. 8. The composition of claim 1 wherein at least 80percent of said particles have a length equal to the average length ±1.0μm and an acicular ratio between approximately 2 and 5, said averagelength being between 0.15 and 0.35 μm.
 9. A process for preparing aparticulate magnetic oxide composition made of particles having a defectspinel structure, considering essentially or (i) iron(III) oxide, (ii)an oxide of at least one bivalent metal selected from the groupconsisting of cobalt, iron, copper, zinc, magnesium, nickel, manganeseand cadmium, and (iii) in the form of an oxide thereof, at least oneadjuvant or substituent selected from the group consisting of an alkalimetal, an alkaline earth metal, boron, aluminum, gallium, germanium,tin, arsenic, indium, antimony, bismuth, lead, the 3d and 4d transitionmetals other than those already mentioned and a rare earth elements,wherein the weight of said at least one bivalent metal representsbetween 1 and 10 percent of the weight of said composition, and whereinthe weight of said at least one adjuvant or substituent representsbetween 0.2 and 5 percent of the weight of said composition, with theproviso that when an alkali metal or tin is present, at least one otheradjuvant or substituent is also present, said process comprising thesteps of heating in air a corresponding mixed oxalate untildecomposition of said mixed oxalate, subjecting the resulting product toa heat treatment, at a rate of 150°-300° C. per hour, in an oxidizingatmosphere at a temperature of 550°-700° C. for a period of from tenminutes to five hours, reducing the resulting product in a hydrogenatmosphere until the alpha-Fe₂ O₃ phase disappears, and then oxidizingthe reduced product by heating in air at a temperature of 100°-500° C.until the desired composition is obtained, and rapidly cooling theresulting composition.
 10. The process of claim 9 which includesperforming an additional heat treatment after the reduction step in aninert atmosphere at a temperature of between 450° and 700° C. so as toincrease the size of the crystals.
 11. The process of claim 9 whereinwhen said adjuvant or substituent is not present in the starting mixedoxalate, said adjuvant or substituent being introduced into said mixedoxalate as an additive.
 12. The process of claim 9 wherein said mixedoxalate and any substituent or adjuvant are decomposed at a temperaturebetween 180° and 300° C. with a temperature rise of less than 20° C. perhour.
 13. The process of claim 9 wherein the reduction step is performedin an atmosphere consisting of a mixture of hydrogen and an inert gascontaining between 8 and 30 percent by volume of hydrogen and containingno water vapor or containing less than 3 percent by volume thereof. 14.The process of claim 9 wherein the reduction is performed at atemperature greater than 330° and lower than 500° C.
 15. The process ofclaim 9 wherein the final oxidation step is performed by heating in airat a temperature of 100°-500° C., said temperature being betweenapproximately 100° and 250° C. when the desired final compositioncontains iron (II) and between 200° and 500° C. in other cases.
 16. Theprocess of claim 9 wherein the oxidation step is performed at atemperature of 100°-250° C. so as to obtain a composition containingbivalent iron.